1. Field of the Art
The present invention relates to a polygon mirror rotor including a polygon mirror, and a polygon mirror scanner motor having such a rotor.
2. Prior Art
A polygon mirror rotor and a polygon mirror scanner motor are widely used in a variety of fields.
FIG. 7 is a cross sectional view showing an arrangement of a conventional polygon mirror rotor of this type. In FIG. 7, reference numeral 1 denotes a ceramic ring of a generally hollow cylindrical shape. The polygon mirror rotor further includes a polygon mirror 3 and upper and lower yokes 4 and 5. When assembling the rotor, the upper yoke 4 is fitted over the ceramic ring 1 by shrink fitting. Then, the bottom surface of the upper yoke 4 and the top surface of the lower yoke 5, which are to be in contact with the upper and lower surfaces of the polygon mirror 3 are finished to flat and smooth surfaces with a high degree of flatness and no surface roughness. Then, the polygon mirror 3 is fitted over the ceramic ring 1 and is sandwiched between the upper and lower yokes 4 and 5, and, then the yokes 4 and 5 are fastened to each other by means of several screws 11 to secure the polygon mirror 3 between them. A rotor magnet 6 forming a part of a driving motor is attached on the bottom surface of the lower yoke 5. The polygon mirror 3 is of a polygonal shape as viewed in plan and has a plurality of mirror surfaces 9 formed therearound.
FIG. 8 is a cross sectional view showing an arrangement of a conventional polygon mirror scanner motor including the above described polygon mirror rotor. As shown, the polygon mirror scanner motor includes a base 17, a shaft 15 fixed on the base 17, a radial shaft sleeve 14 fitted over and fixed to the shaft 15, and a thrust plate 12 fixed to the lower end surface of the shaft sleeve 14. The ceramic ring 1 fitted over the radial shaft sleeve 14 has a cylindrical inner surface in sliding contact with the cylindrical outer surface of the radial shaft sleeve 14, and a lower end surface in sliding contact with the top surface of the thrust plate 12. Another thrust plate 13, which is fixed to the bottom surface of a support plate 19, is fixed to the upper end of the radial shaft sleeve 14. In this arrangement, a radial hydrodynamic bearing is formed between the cylindrical outer surface of the radial shaft sleeve 14 and the cylindrical inner surface of the ceramic ring 1. Further, a pair of thrust hydrodynamic bearings are formed, one between the top surface of the thrust plate 12 and the lower end surface of the ceramic ring 1, and the other between the bottom surface of the thrust plate 13 and the upper end surface of the ceramic ring 1.
The support plate 19 is fixed to the upper end of the shaft 15 by means of a screw 16. A counter magnet 20 is attached to the bottom surface of the support plate 19 at a predetermined position. This counter magnet 20 produces a predetermined magnetic attracting force on the upper yoke 4 made of a ferromagnetic material so as to attract and lift the polygon mirror rotor. There is a stator coil 18 fixed on the base 17. By supplying a current to the stator coil 18, the polygon mirror rotor, and thus the polygon mirror 3, is driven to rotate at high speed.
The above described conventional polygon mirror rotor, however involves the following problems:
(1) When the polygon mirror rotor is driven to rotate at high speed, centrifugal force acts on the polygon mirror 3, and heat produced by rotation raises the temperature of the polygon mirror 3, resulting in a radial expansion or deformation of the polygon mirror 3. However, since several through holes for receiving fastening screws 11 are formed in the polygon mirror 3, the deformation in the polygon mirror is not uniform but differs from area to area depending on whether they are near or far from the through holes, which adversely affects the jitter characteristic of the motor. PA1 (2). The polygon mirror 3 is secured between the upper and lower yokes 4 and 5 by fastening the yokes to each other by means of several screws 11. Thus, the polygon mirror 3 is held in position by the yokes 4 and 5 solely through the frictional force acting between the top surface of the polygon mirror 3 and the lower surface of the upper yoke 4 and between the bottom surface of the polygon mirror 3 and the upper surface of the lower yoke 5. On the other hand, the polygon mirror 3, which is made of aluminum, and the yokes 4 and 5, which may be preferably made of steel, typically have thermal expansion coefficients which are considerably different from each other. Therefore, when a certain temperature change is caused in the polygon mirror rotor by, for example, high speed rotation of the rotor and/or environmental temperature change during operation or storage of the rotor, some slippage may occur between the surfaces in frictional contact with each other (i.e., between the surfaces of the polygon mirror 3 and the associated surfaces of the upper and lower yokes 4 and 5). Any such slippage causes stress in the polygon mirror 3 which tends to adversely affect accuracy in the positioning of the mirror surfaces 9 of the polygon mirror 3 and causes a progressive deterioration in the jitter characteristic of the motor and which further disturbs the balance of the rotor, resulting in an increase in vibration of the rotor. PA1 (3) A change in frictional force acting between the polygon mirror 3 and the yokes 4 and 5 produces stress in the polygon mirror 3, which affects the mirror surfaces 9 of the polygon mirror 3 and may produce some deformation in the mirror surfaces 9. Therefore, all of the surfaces involved in the frictional contacts, i.e., the top and bottom surfaces of the polygon mirror 3 and the associated surfaces of the upper and lower yokes 4 and 5, each having a considerably large area, must be finished into flat and smooth surfaces with a high degree of accuracy. Further, these parts must be carefully assembled to prevent any undesirable stress being induced in the polygon mirror 3. This makes assembly of the polygon mirror rotor very cumbersome and costly. PA1 (4) When the mirror surfaces of the polygon mirror are finished, an oil is used as a lubricating agent.
In the conventional polygon mirror rotor, however, the upper and lower yokes 4 and 5 and the polygon mirror 3 include through holes or threaded holes for receiving the fastening screws 11 therein and the polygon mirror 3 is secured between the upper and lower yokes 4 and 5 by means of several screws 11. Accordingly, the configuration and construction of the polygon mirror rotor are relatively complicated and it is actually impossible to use the oil after assembly of the polygon mirror rotor is completed, since the oil cannot easily be removed from the completed rotor.
Therefore, in the conventional polygon mirror rotor, the mirror surfaces of the polygon mirror must be finished before assembly and, then, the polygon mirror must be assembled in the rotor after completely removing the oil from the polygon mirror.
As a result, a reference plane used for finishing the mirror surfaces of the polygon mirror is different from a reference plane for the assembly or rotation of the polygon mirror rotor.
Therefore, it is very difficult to obtain an ideal or extremely precise center of rotation for all of the mirror surfaces of the polygon mirror and care must be taken during assembling the polygon mirror in the rotor, which makes the production of the polygon mirror rotor very cumbersome and costly.
In view of the foregoing, it is an object of the present invention to provide a polygon mirror rotor which is free from the above shortcomings, does not adversely affect the jitter characteristic of the motor, prevents any progressive deterioration of balance, is easy to manufacture and has superior stability, and a polygon mirror scanner motor including such a polygon mirror rotor.